The principal objective of our work is a further characterization of the DNA molecule at all levels of structure. We have achieved unusually high sensitivity in probing the structure by optical melting. In contrast to published numerical methods, we obtain high resoluton derivative melting curves directly from the differences in absorbance between two identical solutions at a small constant temperature differential, monitored continuously with increasing temperature in a ratio recording spectrophotometer. In one line of study we are characterizing all repetitive sequences in intact and nucleosomal eukaryotic DNAs which are observed as discrete bands superimposed on a broad background in high resolution melting profiles. Analysis of 1) second derivative profiles, 2) spectral dispersion, 3) renaturation time and temperature, and 4) hydroxyapatite fractionation provide means of resolving independent species of repetitive DNA, their G-C composition, sequence length, and number of copies/cell. The specific goal of a second study involves spectral and thermodynamical analysis of high resolution melting of short homogeneous DNAs; particularly those from lambda phi K1974-RFII and PM2 phages; which exhibit considerable fine structure arising from specific sequence dependent subtransitions. Resolution of complex hyperfine detail in the profiles is achieved by analysis of 1) second derivative profiles, 2) spectral dispersion, 3) hysteresis on cooling, and 4) the differential effects of ionic strength.